The present invention relates to an exhaust emission control system of an engine, and particularly to an exhaust emission control system which is provided on an exhaust passage with a NOx catalyst and a SCR (Selective Catalytic Reduction) catalyst which purify NOx in exhaust gas.
Conventionally, NOx storage catalysts which store (occlude) NOx contained in exhaust gas when an air-fuel ratio of the exhaust gas is lean (i.e., λ>1, larger than a theoretical air-fuel ratio) are known. Such NOx storage-reduction catalysts further reduce the stored NOx when the air-fuel ratio is approximately equal to stoichiometric (i.e., λ≈1, approximately equal to the theoretical air-fuel ratio) or is rich (i.e., λ<1, smaller than the theoretical air-fuel ratio). Within a normal operating range of an engine, the engine is operated at the lean air-fuel ratio (λ>1) so as to reduce a fuel consumption, although if this lean operation state continues for a while, the NOx stored amount in the NOx catalyst reaches a limit value and the NOx catalyst can no longer store NOx, which causes NOx to be released. For this reason, the air-fuel ratio is suitably set to be stoichiometric or richer (λ≤1) in order to reduce NOx stored in the NOx catalyst (hereinafter, the control for reducing NOx stored in the NOx catalyst is referred to as “NOx reduction control”). Note that “λ” is an index of the air-fuel ratio expressed with reference to the theoretical air-fuel ratio, and is a so-called air excess ratio.
Further, an exhaust emission control system provided, not only with such a NOx catalyst, but also with a SCR catalyst for selectively reducing and purifying NOx within exhaust gas while using ammonia (NH3) as a reducing agent is recently developed. For example, JP3518398B discloses an art for controlling a SCR catalyst to function when an engine load is high and an engine speed is high, and otherwise controlling a NOx catalyst to function. Further, JP2010-112345A discloses an art for controlling the SCR catalyst to adsorb ammonia generated by the NOx catalyst during a NOx reduction control, and purifying NO using the adsorbed ammonia.
In the art of JP3518398B, since the SCR catalyst has a comparatively high temperature and is active when the engine load and speed are high, NOx is purified by the SCR catalyst instead of the NOx catalyst. Even when the SCR catalyst temperature is comparatively high, if the flow rate of exhaust gas is high, the SCR catalyst cannot suitably purify NOx alone. Therefore, purifying NOx only by the SCR catalyst when the SCR catalyst temperature is comparatively high without considering the flow rate of the exhaust gas may be insufficient and NOx may be discharged.